It is well known in the art to use helical antennas mounted on a structure to allow communication with equipment located at a distance away. More specifically in the aerospace industry, helical antennas such as global coverage antennas are conventionally mounted on spacecraft structure to allow specific communications to and from the ground through a ground station on Earth. Accordingly, spacecraft mounted global coverage antennas are usually located on the conventionally called earth-facing panel of the spacecraft, but can also be mounted on side panels, depending on the respective antenna size and the available room on the panels.
With continuously increasing required antenna gain or other antenna parameters on spacecrafts, the global coverage antennas get larger, such as in the order of a few feet or meters, and depending on their signal frequency range. These large size antennas generate significant mechanical and electrical problems that need to be solved; especially when considering the complex and stringent mechanical and electrical environments the antennas encounter or need to survive. The solution to these problems often requires some trade-offs to be made with the antenna gain, or any other specific requirement the antennas need to meet.
The same concerns apply to large Earth-based helical antennas.
One of the solution known in the art to increase the signal gain of helical antennas is to add a capacitive parasitic element in the form of a tube inserted into the helix formed by the antenna conductor(s) and extending from the free end toward the opposite base end, as taught in U.S. Pat. No. 5,754,146 granted to Knowles et al. on May 19, 1998. Alternatively, the parasitic element can be in the form of a disjointed conductive helix surrounding the conductive helix. This type of parasitic element works generally well for relatively small size helical antennas and cannot realistically be considered for large antennas because of significant problems it would generate.
U.S. Pat. No. 5,923,305 granted to Sadler et al. on Jul. 13, 1999 discloses a dual-band helix antenna with parasitic element positioned either inside or outside of the helix, and may be parallel to the major-axis of the helix, or diagonally relative thereto (when located inside) so as to only be adjacent to two or more windings of the helix. The parasitic element allows the antenna to transmit and receive electrical signals in two widely separated frequency bands.
All known parasitic elements would be cumbersome in large scale applications by adding mass for the parasitic element and its support, and therefore complexity of the overall mechanical and/or electrical design of the antenna, especially if the antenna must be deployed in orbit to be functional.
Accordingly, there is a need for an improved parasitic element for helical antenna.